Female Satin Bowerbirds that can’t quite decide which colorful male has the nicest bower and the sexiest dance moves nervously nibble blue paint.
By Rex Graham
Picking the right male to mate with can be overwhelming. A girl needs a tasty diversion while deciding. Even mammals face similar quandaries. If the female bowerbird isn’t careful, she may enter a male’s bower and copulate before she’s sure.
Every move a displaying male makes is designed to culminate in a single reproductive act. Females at the far opening of his bower must make a decision. Many of them repeatedly nibble on the bluish “paint” he has masticated into a paste from pine needles and applied to the opening of his twig-tunnel of avian love.
Satin Bowerbird nuptials
Darwin was one of many biologists who marveled in wonder at the nuptial antics of the species.
In Queensland, Australia’s subtropical rainforest, female Satin Bowerbirds have lots of glossy, indigo-blue males to choose from. These sedentary, long-lived and intelligent suitors construct stick-tunnel bowers, and sprinkle the carefully groomed ground with blue feathers, berries, shells and other trinket. When a female arrives, he calls and displays with feverish impatience.
Females have good reason to pause. They are vulnerable to involuntary mating after they descend from the trees to the ground.
Gerald Borgia, a University of Maryland Biology Professor and the world’s leading expert on the mating habits of the Satin Bowerbird, said the bowers probably originated to protect females from forced copulations. Each female stands sideways at one end of the bower and watches the male display in the open area beyond the other end of the tunnel of twigs.
Excited males call loudly, flick their wings and crouch. The intensity can escalate quickly as they hop from one side of the bower opening to the other. The most successful males don’t rush: the intensity itself could scare the female. When the female edges into the bower, he quickly moves around behind her and copulates.
The males that breed the most often are able to interpret a female’s comfort level and adjust the intensity of their performance accordingly. This ability to respond to the mental state of other birds is called Theory of Mind. Bowerbirds, parrots and members of the Crow Family exhibit varying degrees of this complex mental prowess.
Nervous paint nibbling
Every move a displaying male Satin Bowerbird makes is designed to culminate in a single reproductive act. While the female at the far opening of his bower is trying to decide whether to take that fateful step inside, she may repeatedly nibble on the bluish “paint” the male has previously applied there.
Males make it by masticating the needles of the Hoop Pine into a paste. “There is now strong evidence that female sampling of paint is important in mate selection,” a team of researchers led by Borgia said in the June 2016 issue of Animal Behavior.
The team found that the frequency with which a female nibbles the paint is an indication of her uncertainty. The most frequent nibblers visit the most bowers and mate with the most males on their rounds.
However, in most cases, a female Satin Bowerbird mates with one male per season. The most successful males have just the right shade of blue feathers, a well-constructed and decorated bower, singing voice and other attributes that add up to a winning combination.
Males mature after at least 7 years old. To increase their odds of breeding, use the singing trick of mimicking the songs of other birds. They cheat. If given the chance, they destroy nearby males’ bowers and steal their decorations. Indeed, Borgia’s group has shown that the males’ performance on an avian versions of intelligence tests also mated with the most females.
The bower is a type of lek, or courtship-display arena. In some species, like Bowerbirds, each male constructs a lek used only by himself. In others, many males may congregate at communal leks. Communal courtship in many insects, amphibians, reptiles and even mammals occur at leks.
Individual male Pronghorn antelopes in the American West use their tightly corralled harems as movable leks. Females can enter and easily sneak away, but males are excluded. Female Pronghorns expend huge amounts of energy wandering from one herd to another in September during the 2 weeks prior to ovulation.
Researchers found that individual radio-collared females invest an unexpectedly high amount of metabolic energy to travel long distances to size up as many males as possible before picking one.
“Our finding calls into question the common assumption in models of mate search that the cost of search is negligible,” researchers with the University of Idaho and the U.S. Fish and Wildlife Service reported in The American Naturalist.
Many polygamous male birds, from the Greater Sage-grouse to the Guanian Cock-of-the-rock, use community leks. These meet-up spots make it easier for females to “sample” males immediately before they lay eggs. Often, the females pick only one male. The most desirable males claim display spots at the center of the lek, which makes the females’ search easier. Once a female selects a male, she may return to the same lek the following year and pick the same guy. This would lower her sampling costs even more.
However, Satin Bowerbirds males don’t cooperate: each builds his own lek. For the females, finding all the leks in a given area to sample the males takes time and patience. They take mental notes on the top males. Maybe it’s too much effort for some females.
Borgia’s team said indecisive females nervously nibble bower paint much more than the more decisive mate-pickers. “We suggest that high rates of tasting are predictive of female uncertainty in mate choice, perhaps due to some females initially having limited information about the quality of potential mates,” the team said.